Method for coating a molybdenum wire with a carbon layer and the coated article



p 1955 P. R. DIJKSTERHUIS ETAL 3,177,094

METHOD FOR COATING A MOLYBDENUM WIRE WITH A CARBQN LAYER AND THE COATED ARTICLE Filed June 15, 1962 Si CI. CCl

INVENTOR RDgKETERHUIS United States Patent 6 2 Claims. (Cl. 117-216) The invention relates to a method of coating at low temperature (approximately 1100 C.) a molybdenum wire with a satisfactorily adhering carbon layer, and to a grid electrode for an electric discharge tube provided with such a wire.

It is known that, at approximately 900 C. to 1000 C., a carbon layer reacts with a molybdenum substratum so that the carbon disappears into the surface. In order to prevent this, it has already been suggested to apply an intermediate layer of a carbide, for example silicon carbide, by coating the core with silica and a carbon-containing substance and by heating in an inert atmosphere to from 1700 to 2000 C. to reduce the oxides and to produce a carbide layer. On this layer, a carbon layer could be applied, for example in the form of powdered graphite. However, it has been found that the molybdenum core becomes very brittle as a result of the strong heating. Consequently, in the known method, the electrodes must generally be formed first prior to the application of the layers of carbide and carbon respectively. A strong heating was then always necessary, since otherwise no intermediate carbide layer was formed. When the carbides were applied to the electrodes in this manner, it was necessary to heat to temperatures above 2000 C. in order to melt the carbides and to obtain a satisfactory adhesion of the carbon layer.

Contrary to all expectations, it has been found, however, that the formation of a satisfactorily adhering intermediate layer under a carbon layer on a molybdenum wire is already possible at a temperature of approximately 1100 C., so that the coated molybdenum wire fully preserves its mechanical properties and can be worked up to grid electrodes without the use of further means, if, according to the invention, the molybdenum wire is first passed through an atmosphere of silicon chloride (SiCl at a temperature of approximately 900 C. and subsequently through an atmosphere of carbon tetrachloride (CCl at approximately 1100 C.

It then appears that after this treatment, the molybdenum core is coated with a very satisfactorily adhering intermediate layer of silicon and thereon a carbon layer and that it can be wound to grids without the use of further provision, since the applied layers are so thin that they substantially do not affect the mechanical properties of the molybdenum. It is assumed that the satisfactory adhesion is obtained in that from the Mo-core to the outside the following layers are found: Mosi Si; SiC; C. It is surprising to find that the reaction between Si and C which generally takes place at a high temperature, can now occur already at approximately 1100 C., which 3,177,094 Patented Apr. 6, 1965 is probably due to the purifying effect of the carbon tetrachloride.

The coating process can be carried out very rapidly. A stay of approximately 1 /2 seconds in each gas atmosphere is suflicient for the wire to be coated with an assembly, of the said layers having a total thickness of approximately 1,u.. The speed of the wire may be from approximately 1000 to 2000 metres per hour, while the length of the furnaces in which the gas atmospheres prevail is approximately 1 m.

At a lower travelling speed, the layers become thicker, as a matter of fact.

The invention will now be described more fully with reference to the accompanying drawing in which:

FIG. 1 diagrammatically illustrates the method;

FIG. 2 shows a grid manufactured with a molybdenum wire coated in accordance with the invention; and

FIG. 3 shows an enlarged sectional view of the wire.

Referring now to FIG. 1, reference numeral 1 denotes the Mo-wire which travels from a supply reel 2 to a supply reel 3 through the furnaces 4 and 5. The wire is heated by means of passage of current through the wire 1, forexample by means of sliding contacts 6. A suitable speed at which the wire passes through the furnaces is approximately 1000 m./hour, the total layer thickness on the wire being approximately 1 Silicon tetrachloride (SiCl is supplied to the furnace 4 and carbon tetrachloride to the furnace 5, both in a gaseous form. The wire thickness may be from'40 to microns.

The grid 9 shown in FIG. 2 has a satisfactory radiation of heat and a low primary and secondary emission as a result of the black carbon layer 8 on the surface of the wire 1, shown in FIG. 3. The intermediate silicon layer is designated by 7. The boundary films (not shown) between the core 1 and the layer 7 and between the layers 7 and 8 probably consist of the compounds MoSi and SiC, as was already indicated.

What is claimed is:

1. A method of coating a molybdenum wire with a satisfactorily adhering carbon layer comprising the steps of passing the molybdenum Wire through an atmosphere of silicon tetrachloride heated to a temperature of approximately 900 C. to deposit a layer of silicon thereon and subsequently passing the silicon-coated molybdenum Wire through an atmosphere of carbon tetrachloride heated to approximately 1100 C. to deposit a coating of carbon thereon.

2. A carbon-coated molybdenum wire for a grid electrode of an electric discharge tube comprising a core of molybdenum, a layer of MoSi adjacent said core, a layer of silicon adjacent the layer of MoSi a layer of silicon carbide adjacent said silicon layer, and an outer layer of carbon covering said silicon carbide layer.

References Cited by the Examiner UNITED STATES PATENTS 876,331 l/08 Clark et al 117231 FOREIGN PATENTS 219,925 2/62 Austria.

RICHARD D. NEVIUS, Primary Examiner. 

1. A METHOD OF COATING A MOLYBDENUM WIRE WITH A SATISFACTORILY ADHERING CARBON LAYER COMPRISING THE STEPS OF PASSING THE MOLYBDENUM WIRE THROUGH AN ATMOSPHERE OF SILICON TETRACHLORIDE HEATED TO A TEMPERATURE OF APPROXIMATELY 900*C. TO DEPOSIT A LAYER OF SILICON THEREON AND SUBSEQUENTLY PASSING THE SILICON-COATED MOLYBDENUM WIRE THROUGH AN ATMOSPHERE OF CARBON TETRACLORIDE HEATED TO APPROXIMATELY 1100*C. TO DEPOSIT A COATING OF CARBON THEREON. 